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Light Emitting Diodes (LED), as the name implies,are diodes that emit light when
forward-biased. LEDs are used to display information. Commonly-used LED displays
are offered as seven-segment displays and dot-matrix displays. The seven-segment
display is arranged in the form of the digit 8 and can display numbers and
hexadecimal characters (0–F).A seven-segment display is a group of eight LEDs
arranged in segments. These segments are generally used to display the numbers 0
through 9 and the letters A through F.

For a certain character, a combination of LED ON and LED OFF is generated to
display the character for a short period of time. The pattern is loaded
alternately to display other characters. For example, to display the number 2,
LEDs a, b, d, e, and g are illuminated.

To control four 7-segment displays, multiplexing canreduce the number of GPIO
pins required.In this setup, the four multiplexed seven-segment displaysare
turned on one at a time to output the appropriatedisplay. Because of the visual
phenomenonknown as persistence of vision, rapid switching of theseven-segment
display can appear as if all four displaysare turned on.

The table illustrates the procedure for displaying afour-digit value in the
multiplexed seven-segmentdisplay. In this example, to be able to display
thevalue 1258, number 8 is displayed first by inputtingthe pattern of number 8.
Next, the NPN transistorconnected to the corresponding seven-segment displayis
switched on in a short period of time while theother transistors remain turned
off. Next, the displayis turned off, and the following pattern is input to
turnon the second transistor. This pattern repeats until thefour digits
alternately display. Rapid switching producesthe illusion that all four
7-segment displays areturned on.